Alkali metal silicate materials, such as sodium aluminosilicates, are well known. Broadly speaking, there are two kinds of alkali metal alumino silicate materials known in the prior art--the natural and the synthetic alkali metal alumino-silicates.
The alkali metal alumino-silicates known as natural zeolites are mined products which are crystalline in nature. Synthetic alkali metal alumino-silicate products are either amorphous or crystalline reaction products. The crystalline synthetic alkali metal alumino-silicates are also called synthetic zeolites. Various types of amorphous synthetic alkali metal alumino-silicates are known as well as synthetic silicas and alumino-silicates.
In order to fully appreciate the present invention it is necessary to draw the appropriate distinctions between the unique compositions of the present invention and the prior art compositions of specific silicas and synthetic silicates referred to in general above.
Zeolites
The prior art description of the nature of zeolites can be found in the U.S. Pat. No. 3,702,886 and is incorporated herein by reference.
Both natural and synthetic zeolites can be broadly classified as crystalline alkali/alkaline earth metal alumino-silicates having unique properties. Synthetic zeolites are ordered, porous crystalline alumino-silicates having a definite crystalline structure within which there are a large number of small cavities which are interconnected by a number of still smaller channels. The cavities and channels are precisely uniform in size. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves include a wide variety of positive ion-containing crystalline alumino-silicates, both natural and synthetic. These alumino-silicates can be described as a rigid three dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra-containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed by formula wherein the ratio of Al to the number of the various cations, such as Ca/2, Sr/2, Na, K, or Li, is equal to unity. One type of cation can be exchanged either in entirety or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the size of the pores in the given alumino-silicate by suitable selection of the particular cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic crystalline alumino-silicates. These alumino-silicates have come to be designated by letter or other convenient symbol, as illustrated by zeolite A (U.S. 2,882,243); zeolite X (U.S. 2,882,244); zeolite Y (U.S. 3,130,007); zeolite K-G (U.S. 3,054 655); zeolite ZK-5 (U.S. 3,247,195); zeolite Beta (U.S. 3,308,069); and zeolite ZK-4 (U.S. 3,314,752), merely to name a few.
Zeolite Identification
Zeolites A and X may be distinguished from other zeolites and silicates on the basis of their x-ray powder diffraction patterns and certain physical characteristics. The composition and density are among the characteristics which have been found to be important in identifying these zeolites.
The basic formula for all crystalline sodium zeolites may be represented as follows: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O
In general, a particular crystalline zeolite will have values for x and y that fall in a definite range. The value x for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. Minor variations in the relative numbers of these atoms does not significantly alter the crystal structure or physical properties of the zeolite. For zeolite A, an average value for x is about 1.85 with the x value falling within the range 1.85.+-.0.5. For zeolite X, the x value falls within the range 2.5.+-.0.5.
The value of y is not necessarily an invariant for all samples of zeolites. This is true because various exchangeable ions are of different size, and, since there is no major change in the crystal lattice dimensions upon ion exchange, the space available in the pores of the zeolite to accommodate water molecules varies.
The average value for y determined for zeolite a is 5.1. For zeolite X it is 6.2.
In zeolites synthesized according to the preferred prior art procedure, the ratio of sodium oxide to alumina should equal one. But if all the excess sodium present in the mother liquor is not washed out of the precipitated product, analysis may show a ratio greater than one, and if the washing is carried too far, some sodium may be ion exchanged by hydrogen, and the ratio will drop below one. It has been found that due to the ease with which hydrogen exchange takes place, the ratio of zeolite A lies in the range of ##EQU1## The ratio of zeolite X lies in the range of ##EQU2## Thus the formula for zeolite A may be written as follows: EQU 1.0.+-.0.2Na.sub.2 0:Al.sub.2 O.sub.3 :1.85.+-.0.5SiO.sub.2 :yH.sub.2 O
Thus the formula for zeolite X may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :2.5.+-.0.5SiO.sub.2 :yH.sub.2 O
y may be any value up to 6 for zeolite A and any value up to 8 for zeolite X.
The pores of zeolites normally contain water. The above formulas represent the chemical analysis of zeolites A and X. When other materials as well as water are in the pores, chemical analysis will show a lower value of y and the presence of other adsorbates. The presence in the crystal lattice of materials volatile at temperatures below about 600 degrees Celsius does not significantly alter the usefulness of the zeolite as an adsorbent since the pores are usually freed of such materials during activation.
Prior Art Patents
Synthetic alkali metal silicates, such as sodium alumino-silicates, are generally produced by the reaction of alum with alkali metal silicates. The resulting product usually has a silica to alumina molar ratio of about 11. Amorphous products of this type are known. For example, amorphous products of this type are sold by the J. M. Huber Corporation under the trademark ZEOLEX.RTM.. Specific examples of these products, as well as methods of their preparation are disclosed in U.S. Pat Nos. 2,739,073; 2,848,346 and 3,582,379.
None of these patents teach or even suggest the synthesis of the unique compositions of the present invention by the hydrothermal reaction between alkali metal silicate bases and clay at preferred molar ratios of silicate base to clay of less than 1.0.
Synthetic silicas are also known which are produced by the reaction of sodium silicates and sulfuric acid at temperatures of about 80 degrees C. The products usually have fixed molar ratios. Various products of this type are known in Satish K. Wason U.S. Pat. Nos. 3,893,840; 4,067,746; 4,122,160 and 4,422,880. Products of this type are sold by J. M. Huber Corporation under the ZEO.RTM., ZEOSYL.RTM., ZEOFREE.RTM. and ZEODENT.RTM. trademarks.
None of the above mentioned patents teach or even suggest the synthesis of the unique compositions of the present invention by the hydrothermal reaction between alkali metal silicate base (B) and clay (C) at preferred molar ratios of B/C or silicate base to clay of less than 1.0. A comparison of the Fourier Transform Infrared (FT-IR) spectra of an amorphous synthetic silicate (ZEOLEX 23), an amorphous silica (Hi-Sil 233) and a synthetic alkali metal alumino-silicate (SAMS) of the instant invention is shown in FIG. 1.
Various zeolite products are also known which are produced synthetically by the reaction of sodium aluminate and sodium silicates at temperatures of less than 100 degrees C. This reaction, in general, proceeds to form an intermediate gel or amorphous material which then crystallizes. Zeolites of this type are sold commercially under the designations, zeolite A, zeolite X, zeolite Y, as well as others. These zeolites find use as absorbents, ion exchange agents, in catalysis and in other areas. A detailed discussion of this art is provided in U.S. Pat. Nos. 4,443,422 and 4,416,805 and is hereby incorporated herein by reference.
None of these patents teach or remotely suggest the synthesis of the unique compositions of the present invention by the hydrothermal reaction between alkali metal silicate base (B) and clay (C) at preferred molar ratios in the batch reaction of B/C or silicate base to clay, of less than 1.0. A comparison of the infrared spectra (FT-IR) of zeolites A, X, and Y with a synthetic alkali metal alumino-silicate (SAMS) prepared by the method described in the instant invention is shown in FIG. 2.
The reaction of sodium silicate with kaolin clays has been studied under various hydrothermal conditions, as reported by Kurbus, et al, Z. Anorg. Allg. Chem., 1977, Volume 429, pages 156-161. These reactions were studied under hydrothermal conditions using essentially equivalent molar ratios of the kaolin and sodium silicate with the reaction being carried out in an autoclave. The products of the reactions, as identified by x-ray, electron microscope, and infrared methods, showed that sodium silicate reacts with kaolin to form an alumino-silica gel or a crystallized zeolite mineral analcime of the formula: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :4SiO.sub.2 :2H.sub.2 O.
In the reaction, the kaolin dissolves and alpha-quartz simultaneously appears in the product of reaction.
Kurbus reference specifically teaches the synthesis of a prior art crystalline zeolite mineral called analcime. This reference does not even remotely suggest the synthesis of the unique compositions of the present invention.
For simplicity, the unique compositions of the instant invention are described as x-ray amorphous materials having attenuated kaolin peaks. The materials will be described in greater detail under the summary of the invention. An FT-IR comparison of analcime with a SAMS composition of the present invention is also given in FIG. 2.
Various reactions of kaolin clays with basic reagents have been studied, including reactions with sodium hydroxide, calcium hydroxide, and the like.
U.S. Pat. Nos. 3,765,825 and 3,769,383 to Hurst, for example, studied the high temperature, high pressure reaction of slurries of clay with alkali metal hydroxides, such as sodium hydroxides. In this reaction, the kaolinite was decomposed and transformed into alumino-silica materials. None of these patents even remotely suggest about the synthesis of the unique composition of the present invention by the hydrothermal reaction between an alkali metal silicate and kaolin clay at preferred molar ratios of silicate to clay of less than 1.0.
Various synthetic amorphous sodium alumino-silicate materials have been produced, as described in U.S. Pat. No. 4,213,874, by the reaction of sodium silicate and sodium aluminate. This patent does not teach or even suggest the synthesis of the unique composition of the present invention by the hydrothermal reaction between an alkali metal silicate and kaolin clay.
In U.S. Pat. No. 3,264,130, a hydroxide of barium or calcium is reacted with a siliceous material. This patent does not teach about the hydrothermal reaction between an alkali metal silicate and kaolin clay.
An amorphous precipitated sodium alumino-silicate pigment is produced in U.S. Pat. No. 3,834,921 by the reaction of sodium silicate and aluminum sulfate. The example of U.S. Pat. No. 3,834,921 teaches about the synthesis of an alumino-silicate pigment of the silica to alumina ratio of about 11.5 The product is produced by reaction of aluminum sulfate and sodium silicate.
None of the above mentioned patents teach or remotely suggest about the synthesis of the unique compositions of the present invention by the hydrothermal reaction between alkali metal silicate base (B) and clay (C) at preferred molar ratios of silicate base to clay, or B/C, of less than 1.0.
In U.S. Pat. No. 4,075,280, zeolite A is produced by the reaction of a calcined clay with sodium hydroxide. This patent teaches about a new process for the preparation of well known prior art zeolite A of well defined x-ray pattern.
Rod-shaped microcrystalline particulates are produced in U.S. Pat. No. 3,837,877 by the reaction of the kaolin clay and an alkali metal hydroxide at molar ratios of hydroxide to clay of at least 2:1. This patent does not even remotely suggest about the synthesis of unique compositions of the instant invention from the hydrothermal reaction between an alkali metal silicate and kaolin clay.
In U.S. Pat. No. 3,784,392, a method is described for preparing finely divided alumino-silicate pigments from kaolin clays by the hydrothermal treatment of kaolin clay dispersions with an alkaline earth metal hydroxide, usually calcium hydroxide. The reaction is carried out using a molar ratio of the hydroxide to the kaolin pigment of at least 1:1 at temperatures of 50 to 200 degrees C. under hydrothermal conditions. The product produced is an amorphous alumino-silicate pigment having increased brightness and having particular utility in coating paper. This patent does not even remotely suggest a reaction between an alkali metal silicate and kaolin to produce unique compositions of the present invention.
None of the prior art patents teach the synthesis of novel alkali metal alumino-silicate compositions described herein. The products of the present invention are unique and their preparation under the disclosed reaction conditions is truly unexpected. For the sake of brevity, the synthetic alkali metal alumino-silicates of the instant invention are referred to as SAMS throughout the body of this patent.
A further background concept necessary to fully appreciate the present invention is that of "structure." As used herein, in relation to alkali metal alumino-silicates, the structure concept is as follows:
It is possible to synthesize alkali metal alumino-silicate or SAMS products with varying structure levels in analogy to the structure definition set forth in U.S. Pat. No. 3,893,840 to S. K. Wason of J. M. Huber Corporation. Since no universally accepted industrial method for particle size determination of synthetic fillers exists and since it is common practice of filler suppliers to perform the rub-out oil absorption test, ASTM-D.281, on their products, the definition of structure is arbitrarily based on the oil absorption values rather than the filler particle size. Conforming to the same definition as in use for silica structure, e.g., S. K. Wason, "Cosmetic Properties and Structure of Fine Particle Synthetic Precipitated Silica," J. Soc. Cosmet. Chem. 29, 497-521 (Aug. 1978), the synthetic alkali metal alumino-silicates or SAMS products are called VHS (very high structure) type when the oil absorption values are above 200 ml/100 g and VLS (very low structure) type when the oil absorption values are below 75 ml/100 g. The classification of the synthetic alkali metal alumino-silicate or SAMS compositions based on "structure" is shown in Table I as it relates to oil absorption.
TABLE I ______________________________________ DEFINITION: SAMS STRUCTURE VERSUS OIL ABSORPTION Oil Absorption Structure Level (ml/100 g) ______________________________________ VHS (Very High Structure) Above 200 HS (High Structure) 175-200 MS (Medium Structure) 125-175 LS (Low Structure) 75-125 VLS (Very Low Structure) Less than 75 ______________________________________
The present invention provides novel synthetic alkali metal alumino-silicate or SAMS compositions and methods for their preparation which are unique and unexpected in view of the knowledge of the prior art involving the reaction of clays and alkali metal silicates.